This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads.
To remain competitive, electrical utility companies continually strive to improve system operation and reliability while reducing costs. To meet these challenges, the utility companies are developing techniques for increasing the life of installed equipment, as well as, diagnosing and monitoring their utility networks. Developing these techniques is becoming increasingly important as the size and demands made on the utility power grid continue to increase. A utility power grid is generally considered to include both transmission line and distribution line networks for carrying voltages greater than and less than about 35 kV, respectively.
Voltage instability on the utility power grid is a critical problem for the utility industry. In particular, when a fault occurs on the transmission grid, momentary voltage depressions are experienced, which may result in voltage collapse or voltage instability on the grid. In general, such a fault appears as an extremely large load materializing instantly on the transmission system. In response to the appearance of this very large load, the transmission system attempts to deliver a very large current to the load (the fault). Detector circuits associated with circuit breakers on the transmission system detect the overcurrent situation immediately (i.e., within a few milliseconds.) Activation signals from the utility protective relays are sent to the circuit breaker which opens the circuit. The mechanical nature of the circuit breakers generally requires 3–6 cycles (i.e., up to 100 msecs) to open. When the breakers open, the fault is cleared. However, opening of the breakers triggers a sequence of events, which in the extreme can cause that portion of the transmission and distribution system to collapse. Specifically, when the breakers open, the voltage is still low (i.e., almost zero) and, because a portion of the transmission system has in effect been removed, the impedance of the system dramatically increases causing the appearance of an artificially high load. In this state the voltage is depressed and the current serving the load sharply increases. The sharp increase in the current generates enormous losses in the transmission and distribution systems. In some cases, because the load and impedance are high, the voltage on the grid may not return to normal, causing long-term voltage depression and the possible voltage collapse of the entire system. The potential for these voltage instability problems are further exacerbated as load requirements on the grid increase.
One approach for addressing this problem is to construct additional transmission lines, thereby negating the effects of the high losses and sharp increase in current flow caused by the opening of the breaker. However, providing such additional lines is expensive and in certain settings extremely difficult.
Various equipment and device solutions have also been developed to address these voltage instability and collapse problems, such as SVCs and STATCOMs as described in greater detail below. In general, such devices remove the losses contributing to the huge increase in current by temporarily injecting power into the system. These losses can be both resistive as well as reactive. To understand the difference between resistive and reactive losses, note that the general expression for average power (when waves of voltage and current are sinusoidal), is
                              V          m                ⁢                  I          m                    2        ⁢    cos    ⁢                  ⁢    θ    ,where Vm and Im represent the peak voltage and current, respectively. Since the maximum value of a sine wave divided by the square root of 2 is the effective value, the equation for average power may be written as:
  P  =                              V          m                          2                    ⁢                        I          m                          2                    ⁢      cos      ⁢                          ⁢      θ        =          VI      ⁢                          ⁢      cos      ⁢                          ⁢      θ      When V is in volts and I is in amperes, the power is expressed in watts. The instantaneous power is:
  p  =            [                                                                  V                m                            ⁢                              I                m                                      2                    ⁢          cos          ⁢                                          ⁢          θ                -                                                            V                m                            ⁢                              I                m                                      2                    ⁢          cos          ⁢                                          ⁢          θcos2ωτ                    ]        +                                        V            m                    ⁢                      I            m                          2            ⁢      sin      ⁢                          ⁢      θsin2ωτ      The first two terms of the right side of this equation represent instantaneous real power. When 2ωτ is an odd multiple of π, the value of the real power is
                    2        ⁢                  V          m                ⁢                  I          m                    2        ⁢    cos    ⁢                  ⁢    θ    =      2    ⁢    VI    ⁢                  ⁢    cos    ⁢                  ⁢    θ  When 2ωτ is a multiple of 2π, the real power is 0. Hence real power in a single-phase circuit fluctuates between 0 and 2VI cos π and has an average value of VI cos π. The third term of the right-hand member of the equation represents what is referred to as instantaneous reactive power, or, preferably, instantaneous reactive volt-amperes. Its equation is
  px  =            (                                                  V              m                        ⁢                          I              m                                2                ⁢        sin        ⁢                                  ⁢        θ            )        ⁢    sin    ⁢                  ⁢    2    ⁢    ωτ  Thus instantaneous reactive volt-amperes fluctuate between
            +                                    V            m                    ⁢                      I            m                          2              ⁢    sin    ⁢                  ⁢    π    ⁢                  ⁢    and    ⁢          -                              V          m                ⁢                  I          m                    2        ⁢    sin    ⁢                  ⁢          π      .      Whereas the average value of the instantaneous reactive volt-amperes is zero, the maximum value is
                    V        m            ⁢              I        m              2    ⁢  sin  ⁢          ⁢      π    .  This is the value referred to when reactive volt-amperes are considered. Hence,
  Px  =                              V          m                          2                    ⁢                        I          m                          2                    ⁢      sin      ⁢                          ⁢      θ        =          VI      ⁢                          ⁢      sin      ⁢                          ⁢      θ      
Reactive volt-amperes are expressed in VARs; a term coined from the first letters of the words “volt amperes reactive”. Reactive volt-amperes considered over a period of time represent oscillations of energy between the source and the load. Their function is to supply the energy for magnetic fields and charging condensers, and to transfer this energy back to the source when the magnetic field collapses or when the condenser discharges. Although reactive volt-amperes, as such, require no average energy input to the generators, they do necessitate a certain amount of generator volt-ampere capacity and thereby limit the available power output of the generators. Reactive power is due to quadrature components of voltage and current and as such represents no average power. These additional losses, which increase the required total real power, are generally supplied by an average energy input to the generators.
Historically, power utilities address severe voltage stability and control issues on transmission and distribution grids with traditional Static VAR Compensator (SVC) and Static Synchronous Compensator (STATCOM) solutions. A STATCOM is a form of an SVC that uses power electronics (e.g., a voltage sourced inverter) to generate the VARs.
Referring to FIG. 1, an SVC 100 is shown to include a phase-controlled TCR (Thyristor Controlled Reactor) 102 and a set of TSCs (Thyristor Controlled Capacitors) 104 connected on the secondary side of a coupling transformer 106. SVC 100 provides reactive power from both TCR 102 and TSCs 104 when a fault is experienced on the utility grid. In particular, TCR 102 and TSCs 104 are connected to transformer via a medium voltage line 108 (12–20 KV). The primary side of transformer 106 is connected to the high voltage transmission line (e.g., >35 KV) 110. In normal operation, a TSC 104 is in the “on” condition all of the time while a TCR 102 is gated on at a specific phase angle every half-line-cycle to cancel out a portion of the capacitive VAR injection. For small phase angles, the conduction time and therefore the inductive VARs is small. For large phase angle approaching 180 degrees, the TCR 102 is essentially “on” the entire half-cycle and more of the capacitive VARs are canceled. A controller (not shown) provides control signals to the TSCs 104 and gating signals to the TCR 102 to allow for infinite control of VAR output from 0–100% depending on system need. Switching of TCR 102 and TSCs 104 occurs very quickly (e.g., within one-half line cycle) using thyristor switches 116. The TCR is sized to provide maximum lagging VARs, while the TSCs may be of the same or different sizes (e.g., 25–100 MVAR) to incrementally introduce capacitive VARs to the system. Thus, TCR 102 serves as a variable VAR compensation device while TSCs 104 serve as fixed but incrementally added/subtracted VAR compensation devices.
In operation, SVC system 100 regulates voltage at its terminal by controlling the amount of reactive power injected into or absorbed from the utility power system. When system voltage is low, SVC 100 generates reactive power (SVC capacitive). When system voltage is high, it absorbs reactive power (SVC inductive). More specifically, SVC 100 rapidly delivers the reactive power to shift the power angle, thereby raising or lowering the voltage on the network. SVC 100 continuously shifts the power angle in response to dynamic power swings on the transmission network due to changing system conditions.
SVC system 100 can also include smaller harmonic filter capacitors 112 (e.g. each 10–30 MVARs) that are always “on” and filter out higher harmonics (e.g., 5th and 7th order harmonics as tuned by inductors 113 in series with capacitors 112) generated by the natural action of the thyristors. SVC system 100 can also be used in conjunction with mechanically-switched capacitors 114 for voltage regulation.
Such static VAR compensators provide capacitive reactance for several reasons. First, utility power systems, particularly at the transmission level, are primarily inductive, due to the length of transmission lines and the presence of numerous transformers. Second, many of the largest loads connected to the utility power system are typically inductive. Large motors used, for example, in lumber mills, rock crushing plants, steel mills, and to drive pumps, shift the power factor of the system away from the desired unity level, thereby decreasing the efficiency of the power system. By selecting the proper amount of capacitance and connection location, the capacitor banks can provide a level of control of the line voltage, power factor, or volt-ampere-reactive (VAR) power. Because most inductive loads operate intermittently and cyclically, the correct compensation is generally applied selectively in response to the varying reactive load on the system.
SVCs and STATCOM systems have the attribute of being capable of providing rapidly changing VARs needed to regulate voltage and quickly drive post-contingency voltages to acceptable levels. The timeframe required for the solution's response is on the order of a few line-cycles of AC power (one line cycle is 16.7 mS for 60 Hz AC power systems) even though it is capable of responding on a sub-cycle basis. However, the primary disadvantage of SVC and STATCOM systems is their high cost.